1. Field of the Invention
This invention relates generally to an electrical vibration type compressor, and particularly to an electrical vibration type compressor wherein a detecting means for detecting the vibration of the drive coil is provided so that a feedback transformer for supplying feedback current is eliminated and the switching operation is properly performed at the top and bottom dead centers of the piston.
2. Description of the Prior Art
Electrical vibration type compressors are used as a compressing means for refrigerators used on board camping cars, trailers, pleasure boats, etc., most of which are powered by small-capacity batteries installed on such vehicles and boats. This necessitates a device which converts d-c current into a-c current as a driving source for the compressor. In this respect, a compressor used for the above-mentioned purposes is required:
(1) TO HAVE AN EXCELLENT OPERATING EFFICIENCY TO MINIMIZE THE CONSUMPTION OF BATTERY POWER SINCE BATTERIES USED ARE GENERALLY OF SMALL CAPACITY.
(2) TO BE SMALL IN SIZE AND LIGHT IN WEIGHT, INCLUDING ITS DRIVE MECHANISM.
To meet these requirements, an electrical vibration type compresor utilizing the resonance phenomenon between the mechanical and electrical vibrations has been developed and is now widely used for the above-mentioned purposes. Although this type of compressor utilizes the resonance between the mechanical and electrical vibrations, it is not always operated in the best resonating condition since the mechanical and electrical vibration cycles tend to disagree due to changes in discharge and suction pressures of refrigerant gas caused by changes in the ambient temperature of the refrigerator and other conditions, changes in the natural vibration cycle of the mechanical vibration system, and ununiform mechanical characteristic of the resonating spring caused in the production process.
Taking into consideration the above points, a drive mechanism has been devised which supplies the drive coil of the compressor with an alternating current produced through the switching operation of transistors, which is performed based on the principle that a transistor is in the saturation region when it is in the switching condition that EQU I.sub.C .ltoreq. h.sub.FE.sup.. I.sub.B ( 1)
where I.sub.C is the collector current; I.sub.B the base current; h.sub.FE the current amplification factor of the switching transistor, while the transistor is shifted to the active region when its switching condition is EQU I.sub.C &gt; h.sub.FE.sup.. I.sub.B ( 2)
with this type of drive mechanism, usually feedback controls the above-mentioned base current I.sub.B through a transformer, but there are some technical problems yet to be improved to meet the requirement (2) above, that is, the small size and light weight. The drive circuit for this type of compressor has been such that a feedback transformer is used to supply a control signal corresponding to the turning-on and -off states of the main semiconductor element to the control electrode of the main semiconductor element so as to cause the main semiconductor element to switch in accordance with the top and bottom dead centers of the piston detected by a detecting means. In other words, a transformer which is energized by the output current of the main semiconductor element has been provided and the feedback winding of the transformer has been designed to supply the control electrode of the main semiconductor element with a feedback current in such a manner that the feedback current is caused to decrease, when the main semiconductor element is turned from the ON state to the OFF state, and thereby the main semiconductor element is caused to shift further to the OFF state, and the feedback current is caused to increase, when the main semiconductor element is turned from the OFF state to the ON state, and thereby the main semiconductor element is caused to shift further to the ON state.
However, using a feedback transformer mentioned above is not favorable since the operating performance is largely dependent on the characteristics of the iron core and the construction of the device tends to be of large size. For this reason, a means to eliminate the transformer has been devised which supplies such a feedback current as to facilitate the switching of the main semiconductor element when the main semiconductor element is turned from the ON state of the OFF state, or from the OFF state to the ON state.